1. Field of the Invention
The present invention relates to a tilt servo apparatus of an optical disc recording/reproducing apparatus and, more particularly, to a liquid-crystal tilt servo apparatus having a liquid crystal device for correcting a tilt error.
2. Description of Related Art
In a recording/reproducing apparatus of an optical disc such as CD (Compact Disc), DVD (Digital Versatile Disc), or the like, generally, a deviation occurs between an optical axis of a laser beam irradiated from an optical pickup and a direction normal to the optical disc at the beam irradiating position due to a warp or the like of the optical disc. An angle of such a deviation is called a tilt angle, and occurs mainly in the direction of radius of the optical disc (hereinafter, referred to as xe2x80x9cradial directionxe2x80x9d), and which causes a coma-aberration or the like of an optical system. When the tilt angle occurs, therefore, signal deterioration such as crosstalk to an adjacent track, jitter, or the like is caused and exerts an adverse influence on the reproducing quality of the optical disc. Particularly, in the case of performing a high density recording like a DVD, since it is necessary to shorten a wavelength of the laser beam and to increase a numerical aperture NA of an objective lens in order to reduce a spot diameter of the laser beam, a margin for the tilt angle decreases. That is, even if the optical disc is slightly inclined, the reproducing quality is greatly deteriorated. To correct an aberration due to the tilt angle during the reproduction of the optical disc, therefore, a disc player is generally provided with a tilt servo mechanism for correcting the tilt error on the basis of a detection signal intensity of the reflected laser beam. As one of such tilt servo systems, there is a liquid-crystal tilt servo apparatus using a liquid crystal device. The principle of the liquid-crystal tilt servo is that a wave front aberration occurring due to a tilt of an optical disc can be compensated by controlling a liquid crystal, which is provided in the optical pickup, to cancel a phase change of the laser beam. A liquid-crystal tilt servo apparatus using a liquid crystal device for compensating the wave front aberration has been disclosed in Japanese Patent Application Kokai No.10-172163 filed by the present applicant.
In the conventional liquid-crystal tilt servo apparatus, a predetermined voltage is applied as a reference voltage to a liquid crystal device and a driving voltage for changing a phase is applied relative to the reference voltage. A driving circuit of the liquid crystal is, therefore, complicated and a DC component is introduced into the liquid crystal and causes deterioration of the liquid crystal.
The present invention is made in consideration of the problems and it is an object of the present invention to provide a liquid-crystal tilt servo apparatus having a simple circuit configuration in which there is no need to generate a reference voltage and deterioration of a liquid crystal can be avoided.
A liquid-crystal tilt servo apparatus according to the present invention comprises: a photodetector for detecting a reflection light beam of a laser beam irradiated to an optical recording medium; a liquid crystal device for correcting a tilt error caused by a tilt angle between an optical axis of the laser beam and a direction normal to the optical recording medium at a laser beam irradiating position; a control apparatus for generating a control signal for controlling the liquid crystal device on the basis of the detected signal intensity of the photodetector; and a driving circuit for generating a driving signal to drive the liquid crystal device on the basis of the control signal, wherein the driving circuit has a DC removing filter for removing a DC component in the driving signal and supplying a resultant signal to the liquid crystal device.
According to another aspect of the present invention, the DC removing filter comprises: a coupling capacitor for coupling the driving circuit and the liquid crystal device; and a resistor connected between an output terminal of the coupling capacitor and a ground terminal, wherein the capacitance of the coupling capacitor is larger than the internal capacitance of the liquid crystal device and the resistance of the resistor is smaller than the internal resistance of the liquid crystal device.